MAC And Broadcast Protocol Design And Optimization Based On 802.11p Standard For VANETs

VANET (Vehicular Ad hoc Network) is a special kind of MANET (Mobile Ad hoc Network). As one of the most crucial parts of Intelligent Transport Systems (ITS), VANET has been applicator widely in traffic information warning, driving abidance, entertainments and so on.VANET, as a wireless network, is vulnerable due to its high mobility and large scale of network. It is important to research how to ensure the reliability of the message transmission in VANETs. Based on analyzing the characteristics of IEEE 802.11p/1609 standards,A self-adaptive back-off algorithm and a broadcasting scheme for emergency messages are proposed to resolve the unreliability problem of the message dissemination. The main contributions of this thesis are as follows:Firstly, according to the main features of vehicular networks such as the high speed and dynamic network topology, the ratio of the loss packets is increasing. In order to resolve this problem, a relative-speed-based self-adaptive back-off algorithm (RSBA) is proposed in this paper. Each vehicle adjusts the value of the minimum contention window according to the deviation of the node speed from the average speed of the neighbors. Therefore the back off mechanism gets better, and the requirements of the transmission could be adapted. Through simulation the RSBA is proved to decrease the ratio of dropped packets and help improving the performance of the IEEE 802.11p MAC protocol.Secondly, according to the main performance goals of disseminating emergency messages such as high reliability, fast dissemination, a power-control broadcast (PCB) scheme for emergency messages is proposed based on studying the IEEE 802.11p and IEEE 1609 standards for vehicular communications. Transmission strategy is adjusted based on vehicles'situation and the receive power of recently broadcast packet from a sender node, and re-dissemination power in short-range for reliable broadcast is adapted based on the observed channel status. The simulation results demonstrate high delivery ratio and fast emergency messages dissemination while avoiding congestion.Finally, through the design and preparation of the required network protocol module and the vehicle control module, the 802.11e-based vehicle network simulators are designed and implemented. Using the NS-2 simulator and NCTUns network simulator, the network operating conditions is simulated, and the performance of these two simulators are summarized and compared.